Desilicification Rims of Zircon Xenocrysts Record the Timing of Kimberlite Emplacement

Kimberlites are deep and relatively rare mantle-derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout Earth's history may reveal secular changes in global geodynamics. However, alteration and abundant exotic material (i.e., xenoliths and xenocrysts) of different ages, both typical of kimberlites, often hinder reliable dating of these rocks. In this study, we investigated zircon xenocrysts and their replacement by desilicification reaction rims (DSRs), which consist mainly of baddeleyite, in the metamorphosed Kimozero kimberlites from the Karelian Craton, Russia. Similar DSRs have been identified previously on mantle-derived zircons in numerous kimberlites, but the precise timing and location of formation of these rims remain unclear. U-Pb isotope dating of mantle zircon xenocrysts in the Kimozero kimberlites indicates that these rocks were emplaced at ca. 1.97 Ga. DSRs with similar textures and baddeleyite chemistry occur on both the 1.97 Ga mantle and 2.70-2.38 Ga crustal zircon xenocrysts in these kimberlites. Pb-Pb isotope dating of the DSRs yields the same age (i.e., ca. 1.97 Ga) as the mantle zircons. The data constrain the formation of DSRs to crustal levels and, more importantly, indicate that these rims record the timing of kimberlite emplacement. Therefore, we suggest that such baddeleyite reaction rims are a new and promising material for kimberlite geochronology.